A microcontrol unit for a superpipelined, superscalar microprocessor

Abstract: A superscalar, superpipelined microprocessor includes storage circuitry to store an instruction, memory circuitry addressable by a microaddress to output a microinstruction in response to the microaddress, sequencing circuitry coupled to provide the microaddress to the memory circuitry, and decode circuitry coupled to the storage circuitry to detect whether the instruction stored in the storage circuitry comprises a single clock instruction, before the memory circuit outputs the microinstruction, and to indicate such a detection to the sequencing circuitry. Features for single microROM support for a dual pipeline architecture and execution of condition dependent instructions over a plurality of execution stages are further included within the microprocessor.

Description: Background to the Invention 1. Field of the Invention This invention relates to a processor and a method of operating a processor. 2. Description of Related Art In the design of a microprocessor, instruction throughput, i.e., the number of instructions executed per second, is of primary importance. The number of instructions executed per second may be increased by various means. The most straightforward technique for increasing instruction throughput is by increasing frequency at which the microprocessor operates. Increased operating frequency, however, is limited by fabrication techniques and also results in the generation of excess heat. An example of a known microprocessor design is to be found in US Patent No. 4,415,969. Thus, modern day microprocessor designs are focusing on increasing the instruction throughput by using design techniques which increase the average number of instructions executed per clock cycle period. One such technique for increasing instruction throughput is "pipelining". Pipelining techniques segment each instruction flowing through the microprocessor into several portions, each of which can be handled by a separate stage in the pipeline. Pipelining increases the speed of a microprocessor by overlapping multiple instructions in execution. For example, if each instruction could be executed in six stages, and each stage required one clock cycle to perform its function, six separate instructions could be simultaneously executed (each executing in a separate stage of the pipeline) such that one instruction was completed on each clock cycle. In this ideal scenario, the pipelined architecture would have an instruction throughput which was six times greater than the non-pipelined architecture, which could complete one instruction every six clock cycles. A second technique for increasing the speed of a microprocessor is by designing it to be a "superscalar." In a superscalar architecture, more than one instruction is issued per clock cycle. If no instructions were dependent upon other instructions in the flow, the increase in instruction throughput would be proportional to the degree of scaleability. Thus, if an architecture were superscalar to degree 2 (meaning that two instructions issued upon each clock cycle), then the instruction throughput in the machine would double. A microprocessor may be both superpipelined (an instruction pipeline with many stages is referred to as "superpipelined") and superscalar to achieve a high instruction throughput. However, the operation of such a system in practice is far from the ideal situation where each instruction can be neatly executed in a given number of pipe stages and where the execution of instructions is not interdependent. In actual operation, instructions have varying resource requirements, thus creating interruptions in the flow of instructions through the pipeline. Further, the instructions typically have interdependencies; for example, an instruction which reads the value of a register is dependent on a previous instruction which writes the value to that same register - the second instruction cannot execute until the first instruction has completed its write to the register. Consequently, while superpipelining and superscalar techniques can increase the throughput of a microprocessor, the instruction throughput is highly dependent upon the implementation of the superpipelined, superscalar architecture. One particular problem is reducing the number of wasted clock cycles following an instruction requiring only one clock cycle to perform its execution stage. Such an instruction executes its corresponding microinstruction in a single clock cycle, and is hereinafter referred to as a "single clock instruction". Particularly, under current pipeline architectures, after a single clock instruction, additional clock cycles are expended before the next instruction's microaddress is obtained. Consequently, the next successive instruction corresponding to that microaddress is detained from advancing through the pipeline. Therefore, a need has arisen for a microprocessor architecture wherein the number of clock cycles before obtaining the next microaddress following a single clock instruction is reduced over those in the prior art. Further, a need has arisen for a microprocessor architecture wherein the overall size or number of array space needed to accommodate multiple pipelines is reduced over those in the prior art. Further yet, a need has arisen for a microprocessor architecture wherein the number of clock cycles before completing, or advancing, the execution of a condition dependent instruction is reduced over those in the prior art. Summary of the Invention In accordance with a first aspect of the invention there is provided a processor comprising a decoder for storing instructions to be decoded; a memory responsive to a micro-address to output a micro-instruction associated with an instruction stored in said decoder; and sequencer circuitry coupled to provide a micro-address to said memory; characterised by a detector coupled to the decoder and operable to detect whether or not an instruction comprises a single clock instruction before said memory outputs the micro-instruction associated with that instruction, said detector being responsive to detection of a single clock instruction to input a control signal to the sequencer circuitry to cause the sequencer circuitry to issue, independently of said memory, another micro-address associated with another instruction. In accordance with a second aspect of the invention there is provided a method of operating a processor, the method comprising the steps of: storing instructions; outputting a micro-address from a sequencer to address a memory thereby causing the memory to issue a micro-instruction associated with a stored instruction; characterised by detecting, before said memory outputs the micro-instruction associated with an instruction, whether or not that instruction is a single clock instruction; and in response to the detection of a single clock instruction, inputting a control signal to the sequencer to provide, independently of said memory, another micro-address associated with another instruction. In accordance with one embodiment of the invention there is provided a microprocessor having an expedited execution of condition dependent instructions, a single clock instruction decode architecture, and a single microROM operation for supporting a dual pipeline architecture. In an embodiment of the invention there is provided an improved microcontrol unit and methodology, wherein change of flow instructions are expedited by issuing a first microinstruction having a control signal and a base address from memory and determining a secondary address by evaluating a plurality of predetermined data. The base address and the secondary address are combined to form a destination address, wherein the destination address identifies a second microinstruction in the memory to execute a successive stage for the change of flow instruction. As an alternative to the secondary address, a completion signal, indicating the completion of the execution stages of the change of flow instruction, may be issued by evaluating a plurality of predetermined data. Decode circuitry may be provided to detect whether an instruction is a single clock instruction and, in response thereto, signals sequencing circuitry to expedite the next microinstruction. A multiple pipeline microprocessor can be provided that outputs micromemory instructions from a single micromemory. The processor and methodology of the present invention make it possible to achieve numerous technical advantages. Although generally the technical advantages equate to enhanced processor throughput, particular embodiments of different aspects of the invention may result in one or more of the following advantages: (i) reduction in the number of accesses to the microinstruction memory while executing a condition dependent instruction and by determining the completion of a condition dependent instruction; (ii) pipelining the access to, and output from, the processor micromemory--requiring less hardware, power consumption, surface area, and costs; and (iii) the reduction in the delay following a single clock instruction. Still other technical advantages should be readily ascertainable to a person having skill in the art with reference to the following description, claims, and drawings. Brief Description of the Drawings For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: Figure 1a illustrates a block diagram of the overall microprocessor; Figure 1b illustrates a generalized block diagram of the instruction pipeline stages; Figure 2 illustrates a block diagram of a processor system using the microprocessor; Figure 3 illustrates a timing diagram showing the flow of instructions through the pipeline stages; Figure 4a illustrates a simplified block diagram including the microsequencer and microROM of the microprocessor; Figure 4b illustrates a timing diagram showing the generation of a microaddress and the resultant microinstruction output by the microROM; Figure 4c illustrates a timing diagram showing the preferred timing of one aspect of the present invention in generating a microinstruction and obtaining the next successive microaddress; Figure 4d illustrates a timing diagram showing the wasted clock cycle in the prior art between generation of a microinstruction and obtaining the next successive microaddress; Figure 5 illustrates a block diagram of Figure 4a including a detailed illustration of each instruction register and its respective programmable logic array; Figure 6 illustrates the preferred apparatus for the microROM circuit; Figure 7 illustrates a timing diagram for both the x- and y-accessing of the microROM array of the microROM circuit; and Figure 8 illustrates a portion of the components of Figure 4a, with additional apparatus for expediting change of flow (COF) or other condition dependent instructions. Detailed Description of the Preferred Embodiments The detailed description of an exemplary embodiment of the microprocessor of the present invention is organized as follows: 1. Exemplary Processor System 1.1. Microprocessor. 1.2. System 2. Generalized pipeline architecture 3. Single clock instruction decode architecture 4. Single microROM operation for supporting dual pipeline architecture 5. Expediting Execution of Condition Dependent Instructions 6. Conclusion This organizational table, and the corresponding headings used in this detailed description, are provided for the convenience of reference only. Detailed description of conventional or known aspects of the microprocessor are omitted as to not obscure the description of the invention with unnecessary detail. 1. Exemplary Processor System. The exemplary processor system is shown in Figures 1a, 1b, and Figure 2. Figures 1a and 1b respectively illustrate the basic functional blocks of the exemplary superscalar, superpipelined microprocessor along with the pipe stages of the two execution pipelines. Figure 2 illustrates an exemplary processor system (motherboard) design using the microprocessor. 1.1. Microprocessor. Referring to Figure 1a, the major sub-blocks of a microprocessor 10 include: (a) central processing unit (CPU) core 20, (b) prefetch buffer 30 , (c) prefetcher 35, (d) branch processing unit (BPU) 40, (e) address translation unit (ATU) 50, and (f) unified 16 Kbyte code/data cache 60, including TAG RAM 62. A 256 byte instruction line cache 65 provides a primary instruction cache to reduce instruction fetches to the unified cache, which operates as a secondary instruction cache. An onboard floating point unit (FPU) 70 executes floating point instructions issued to it by the CPU core 20. The microprocessor uses internal 32-bit address and 64-bit data buses ADS and DATA, respectively. A 256 bit (32 byte) prefetch bus (PFB), corresponding to the 32 byte line size of the unified cache 60 and the instruction line cache 65, allows a full line of 32 instruction bytes to be transferred to the instruction line cache in a single clock. Interface to external 32 bit address and 64 bit data buses is through a bus interface unit (BIU). The CPU core 20 is a superscalar design with two execution pipes X and Y. It includes an instruction decoder 21, address calculation units 22X and 22Y, execution units 23X and 23Y, and a register file 24 with 32 32-bit registers. An AC control unit 25 includes a register translation unit 25a with a register scoreboard and register renaming hardware. A microcontrol unit 26, including a microsequencer and microROM, provides execution control. Writes from CPU core 20 are queued into twelve 32 bit write buffers 29 --write buffer allocation is performed by the AC control unit 25. These write buffers provide an interface for writes to the unified cache 60 -- noncacheable writes go directly from the write buffers to external memory. The write buffer logic supports optional read sourcing and write gathering. A pipe control unit 28 controls instruction flow through the execution pipes, including: keeping the instructions in order until it is determined that an instruction will not cause an exception; squashing bubbles in the instruction stream; and flushing the execution pipes behind branches that are mispredicted and instructions that cause an exception. For each stage, the pipe control unit keeps track of which execution pipe contains the earliest instruction, provides a "stall" output, and receives a "delay" input. BPU 40 predicts the direction of branches (taken or not taken), and provides target addresses for predicted taken branches and unconditional change of flow instructions (jumps, calls, returns). In addition, it monitors speculative execution in the case of branches and floating point instructions, i.e., the execution of instructions speculatively issued after branches which may turn out to be mispredicted, and floating point instructions issued to the FPU which may fault after the speculatively issued instructions have completed execution. If a floating point instruction faults, or if a branch is mispredicted (which will not be known until the EX or WB stage for the branch), then the execution pipeline must be repaired to the point of the faulting or mispredicted instruction (i.e., the execution pipeline is flushed behind that instruction), and instruction fetch restarted. Pipeline repair is accomplished by creating checkpoints of the processor state at each pipe stage as a floating point or predicted branch instruction enters that stage. For these checkpointed instructions, all resources (programmer visible registers, instruction pointer, condition code register) that can be modified by succeeding speculatively issued instructions are checkpointed. If a checkpointed floating point instruction faults or a checkpointed branch is mispredicted, the execution pipeline is flushed behind the checkpointed instruction -- for floating point instructions, this will typically mean flushing the entire execution pipeline, while for a mispredicted branch there may be a paired instruction in EX and two instructions in WB that would be allowed to complete. For the exemplary microprocessor 10, the principle constraints on the degree of speculation are: (a) speculative execution is allowed for only up to four floating point or branch instructions at a time (i.e., the speculation level is maximum 4), and (b) a write or floating point store will not complete to the cache or external memory until the associated branch or floating point instruction has been resolved (i.e., the prediction is correct, or floating point instruction does not fault). The unified cache 60 is 4-way set associative (with a 4k set size), using a pseudo-LRU replacement algorithm, with write-through and write-back modes. It is dual ported (through banking) to permit two memory accesses (data read, instruction fetch, or data write) per clock. The instruction line cache is a fully associative, lookaside implementation (relative to the unified cache), using an LRU replacement algorithm. The FPU 70 includes a load/store stage with 4-deep load and store queues, a conversion stage (32-bit to 80-bit extended format), and an execution stage. Loads are controlled by the CPU core 20, and cacheable stores are directed through the write buffers 29 (i.e., a write buffer is allocated for each floating point store operation). Referring to Figure 1b, the microprocessor has seven-stage X and Y execution pipelines: instruction fetch (IF), two instruction decode stages (ID1, ID2), two address calculation stages (AC1, AC2), execution (EX), and write-back (WB). Note that the complex ID and AC pipe stages are superpipelined. The IF stage provides a continuous code stream into the CPU core 20. The prefetcher 35 fetches 16 bytes of instruction data into the prefetch buffer 30 from either the (primary) instruction line cache 65 or the (secondary) unified cache 60. BPU 40 is accessed with the prefetch address, and supplies target addresses to the prefetcher for predicted changes of flow, allowing the prefetcher to shift to a new code stream in one clock. The decode stages ID1 and ID2 decode the variable length X86 instruction set. The instruction decoder 21 retrieves 16 bytes of instruction data from the prefetch buffer 30 each clock. In ID1, the length of two instructions is decoded (one each for the X and Y execution pipes) to obtain the X and Y instruction pointers -- a corresponding X and Y bytes-used signal is sent back to the prefetch buffer (which then increments for the next 16 byte transfer). Also in ID1, certain instruction types are determined, such as changes of flow, and immediate and/or displacement operands are separated. The ID2 stage completes decoding the X and Y instructions, generating entry points for the microROM and decoding addressing modes and register fields. During the ID stages, the optimum pipe for executing an instruction is determined, and the instruction is issued into that pipe. Pipe switching allows instructions to be switched from ID2X to AC1Y, and from ID2Y to AC1X. For the exemplary embodiment, certain instructions are issued only into the X pipeline: change of flow instructions, floating point instructions, and exclusive instructions. Exclusive instructions include: any instruction that may fault in the EX pipe stage and certain types of instructions such as protected mode segment loads, string instructions, special register access (control, debug, test), Multiply/Divide, Input/Output, Push All/Pop All (PUSH/POPA), and task switch. Exclusive instructions are able to use the resources of both pipes because they are issued alone from the ID stage (i.e., they are not paired with any other instruction). Except for these issue constraints, any instructions can be paired and issued into either the X or Y pipe. The address calculation stages AC1 and AC2 calculate addresses for memory references and supply memory operands. The AC1 stage calculates two 32 bit linear (three operand) addresses per clock (four operand addresses, which are relatively infrequent, take two clocks). Data dependencies are also checked and resolved using the register translation unit 25a (register scoreboard and register renaming hardware) -- the 32 physical registers 24 are used to map the 8 general purpose programmer visible logical registers defined in the X86 architecture (EAX, EBX, ECX, EDX, EDI, ESI, EBP, ESP). The AC unit includes eight architectural (logical) registers (representing the X86 defined register set) that are used by the AC unit to avoid the delay required to access in AC1 the register translation unit before accessing register operands for address calculation. For instructions that require address calculations, AC1 waits until the required data in the architectural registers is valid (no read after write dependencies) before accessing those registers. During the AC2 stage, the register file 24 and the unified cache 60 are accessed with the physical address (for cache hits, cache access time for the dual ported unified cache is the same as that of a register, effectively extending the register set) -- the physical address is either the linear address, or if address translation is enabled, a translated address generated by the ATU 50. Translated addresses are generated by the ATU 50 from the linear address using information from page tables in memory and workspace control registers on chip. The unified cache is virtually indexed and physically tagged to permit, when address translation is enabled, set selection with the untranslated address (available at the end of AC1) and, for each set, tag comparison with the translated address from the ATU 50 (available early in AC2). Checks for any segmentation and/or address translation violations are also performed in AC2. Instructions are kept in program order until it is determined that they will not cause an exception. For most instructions, this determination is made during or before AC2 -- floating point instructions and certain exclusive instructions may cause exceptions during execution. Instructions are passed in order from AC2 to EX (or in the case of floating point instructions, to the FPU) --integer instructions that may still cause an exception in EX are designated exclusive, and therefore are issued alone into both execution pipes, handling exceptions in order is ensured. The execution stages EXX and EXY perform the operations defined by the instruction. Instructions spend a variable number of clocks in EX, i.e., they are allowed to execute out of order (out of order completion). Both EX stages include adder, logical, and shifter functional units, and in addition, the EXX stage contains multiply/divide hardware. The WB stage updates the register file 24, condition codes, and other parts of the machine state with the results of the previously executed instruction. The register file is written in Phase 1 (PH1) of WB and read in Phase 2 (PH2) of AC2. 1.2. System. Referring to Figure 2, for the exemplary embodiment, microprocessor 10 is used in a processor system that includes a single chip memory and bus controller 82. The memory/bus controller 82 provides the interface between the microprocessor and the external memory subsystem -- level two cache 84 and main memory 86 -- controlling data movement over the 64 bit processor data bus PD (the data path is external to the controller which reduces its pin count and cost). Controller 82 interfaces directly to the 32-bit address bus PADDR, and includes a one bit wide data port (not shown) for reading and writing registers within the controller. A bi-directional isolation buffer 88 provides an address interface between microprocessor 10 and VL and ISA buses. Controller 82 provides control for the VL and ISA bus interface. A VL/ISA interface chip 91 (such as an HT321) provides standard interfaces to a 32 bit VL bus and a 16 bit ISA bus. The ISA bus interfaces to BIOS 92, keyboard controller 93, and I/O chip 94, as well as standard ISA slots 95. The interface chip 91 interfaces to the 32 bit VL bus through a bi-directional 32/16 multiplexer 96 formed by dual high/low word [31:16]/[15:0] isolation buffers. The VL bus interfaces to standard VL slots 97, and through a bi-directional isolation buffer 98 to the low double word [31:0] of the 64 bit processor data (PD) bus. 2. Generalized pipeline architecture. Figure 3 illustrates the flow of four instructions per pipeline, showing the overlapping execution of the instructions, for a two pipeline architecture. Additional pipelines and additional stages for each pipeline could also be provided. In the preferred embodiment, the microprocessor 10 uses an internal clock 122 which is a multiple of the system clock 124. In Figure 3, the internal clock is shown as operating at two times the frequency of the system clock. During the first internal clock cycle 126, the ID1 stage operates on respective instructions X0 and Y0. During internal clock cycle 128, instructions X0 and Y0 are in the ID2 stage (X0 being in ID2x and Y0 being in ID2y) and instructions X1 and Y1 are in the ID1 stage. During internal clock cycle 130, instructions X2 and Y2 are in the ID1 stage, instructions X1 and Y1 are in the ID2 stage (X1 being in ID2x and Y1 being in ID2y) and instructions X0 and Y0 are in the AC1 stage (X0 being in AC1x and Y0 being in AC1y). During internal clock cycle 132, instructions X3 and Y3 are in the ID1 stage, instructions X2 and Y2 are in the ID2 stage, instructions X1 and Y1 are in the AC1 stage and instructions X0 and Y0 are in the AC2 stage. The instructions continue to flow sequentially through the stages of the X and Y pipelines. As shown in clocks 134-140, the execution portion of each instruction is performed on sequential clock cycles. This is a major advantage of a pipelined architecture - the number of instructions completed per clock is increased, without reducing the execution time of an individual instruction. Consequently a greater instruction throughput is achieved with greater demands on the speed of the hardware. The instruction flow shown in Figure 3 is the optimum case. As shown, each pipe stage completes on time and no stage requires more than one clock cycle. In an actual machine, however, one or more stages may require additional clock cycles to complete thereby changing the flow of instructions through the other pipe stages. Furthermore, the flow of instructions through one pipeline may be dependent upon the flow of instructions through the other pipeline. A number of factors may cause delays in various stages of one or all of the pipelines. For example, an access to memory may miss in the memory cache, thereby preventing access of the data in the time required to process the instruction in one clock. This would require that either, or both, sides of the AC1 stage to delay until the data was retrieved from main memory. For a particular stage, other stages of the pipeline may be using a needed resource, such as a multiplier, which is only in one of the execution stages in the illustrated embodiment. In this case, the stage must delay until the resource is available. Data dependencies can also cause delays. If an instruction needs the result from a previous instruction, such as an ADD, it must wait until that instruction is processed by the execution unit. Other delays are caused by "multi-box" instructions; i.e., instructions which are implemented using multiple microinstructions, and therefore require more than one clock cycle to complete. These instructions stop the flow of subsequent instructions through the pipeline at the output of the ID2 stage. The flow of instructions through the pipeline is controlled by the pipe control unit 28. In the preferred embodiment, a single pipe control unit 28 is used to control the flow of instructions through both (or all) of the pipes. To control the flow of instructions through the pipes, the pipe control unit 28 receives "delay" signals from the various units comprising the pipelines 102 and 104, and issues "stall" signals to the various units. Although a single pipe control unit 28 is used for both X and Y pipelines, the pipelines themselves are controlled independent of one another. In other words, a stall in the X pipeline does not necessarily cause a stall in the Y pipeline. 3. Single clock instruction decode architecture. Figure 4a depicts a block diagram having certain components involved in the generation of microaddresses and the resulting microinstructions. While the blocks of Figure 4a derive from those in Figure 1, it should be understood that the method of the present invention is not necessarily limited by the particular locations of specific components, and one skilled in the art may relocate various functionality without departing from the inventive scope. With reference to Figure 4a, instructions for both the x- and y-pipelines are coupled to an instruction decoder 21. Decoder 21 receives the x- and y-instructions and provides both microaddresses and control to microcontrol unit 26. More particularly, the microaddresses and control are coupled to a microsequencer circuit 146 within microcontrol unit 26. In the preferred embodiment, microsequencer circuit 146 includes independent microsequencers 146x and 146y for performing like functions with respect to the x- and y-pipelines. Microsequencer circuit 146 also receives both control and microaddresses from other sources (not shown) throughout the microprocessor. Microsequencer circuit 146 is coupled to provide addresses, hereinafter referred to as microaddresses, to address the memory (shown and described later) in a microROM circuit 148. Figure 4b illustrates the timing of microaddress generation, microinstruction generation, and microinstruction execution by the blocks of Figure 4a. Note that the timing of Figure 4b derives from Figure 3, but with greater emphasis on the stages corresponding to the operations of the blocks of Figure 4a. Note also that the stages of Figure 4b illustrate the sequence of an x- and y-instruction through four internal clock cycles evenly numbered 150 through 156. Clock cycles 150 through 156 include the ID2, AC1, AC2 and EX stages. Note also that each clock cycle includes two phases, denoted 1 and 2. The general operation of the blocks of Figure 4a and the timing of Figure 4b, both in connection with a single clock instruction, are as follows. As mentioned in connection with Figure 3, during stage ID1, instruction decoder 21 determines the number of bytes in the single clock instruction and the types of fields included in the instruction. Decoder 21 also provides entry point microaddresses derived from the instructions for addressing microROM circuit 148. During stage ID2, microsequencer circuit 146 outputs the next microaddress for microROM circuit 148 for both the x- and y-instruction. For purposes of explanation, the next microaddresses are labeled as µsx 1 and µsy 1 , respectively, with µs indicating output from the microsequencer. Note that µsx 1 , is output by microsequencer circuit 146 during the first phase, 1, of stage ID2. Similarly, µ sy 1 is output by microsequencer circuit 146 during the second phase, 2, of stage ID2. Microsequencer circuit 146 operates as a sophisticated multiplexer; thus, in response to its control signals, microsequencer circuit 146 chooses from its microaddress inputs to output the next microaddress. In the preferred embodiment, the next microaddress selected and output by microsequencer circuit 146 may be one of eight different microaddresses. The provision of some of the microaddresses is specifically described throughout this document, with it understood that the remainder fall within the general indication in Figure 4a of an input providing microaddresses to multiplexer circuit 146. Briefly, the seven different microaddresses are: (1) instruction entry point -- this microaddress is provided by decoder 21 and is selected when a new instruction is to enter the EX stage; (2) exception PLA -- this microaddress is selected to service an interrupt or an exception; (3) NOP -- this microaddress is selected when there are no instructions in the corresponding pipeline to be executed; (4) micro return -- this microaddress is selected from the top of the return stack; (5) microROM next microaddress -- this microaddress is selected from the current output by microROM circuit 148 and may be used to select the next sequential pair of microinstructions, or the target microaddress of certain calls or conditional microbranches; (6) ntaken: -- this address is selected when certain conditional microbranches are not taken; and (7) change of flow microaddress -- this microaddress is a concatenated address formed in part from the microROM next microaddress and in part from an instruction decode circuit. The preferred method of generating this microaddress is discussed in detail in section 5, below. The microaddresses are applied by microsequencer circuit 146 to microROM circuit 148, which outputs a microinstruction to carry out whatever functionality is called for by the corresponding microaddress. Thus, returning to Figure 4b, immediately following the output of microaddress µsx 1 , that is, following 1 of stage ID2, µsx 1 is applied to microROM circuit 148. As described in greater detail in section 4, below, between 2 of stage ID2 and by the end of 1 of stage AC1, an array within microROM circuit 148 generates and outputs the corresponding microinstruction, labeled µrx 1 . Note, for clarity, µr suggests output from microROM circuit 148. Moreover, immediately following the output of microaddress µsy 1 (following 2 of stage ID2), µsy 1 is applied to microROM circuit 148. Also as detailed in section 4, below, between 1 and by the end of 2 of stage AC1, the array of microROM circuit 148 generates and outputs the corresponding microinstruction, labeled µry 1 . Stage AC2 occurs in clock cycle 154 and the microinstructions corresponding to instructions X1 and Y1, µrx 1 and µry 1 , are presented to the applicable components of the microprocessor. Note in the preferred embodiment that each pipeline has its own respective address calculation stage and, thus, the x- and y-microinstructions are presented accordingly. Thus, clock cycle 154 is illustrated as applying to both "x 1 & y 1 " during the entire duration of the AC2 stage. Moreover, in the AC2 stage, each respective microinstruction is decoded in preparation for execution. Finally, stage EX occurs in clock cycle 156, where the decoded instructions are applied to respective x- and y-pipeline ALUs, thereby executing the functions specified by the presented microinstructions. Like the AC2 stage, the x- and y-operations occur independently of one another and, hence, are illustrated as applying to both "x 1 & y 1 " during the entire duration of the EX stage. Note that because Figure 4b corresponds to the flow of a single clock instruction, only a single clock cycle 156 is necessary for the execution stage of the instruction. Multiple clock instructions require additional execution stages. Having explained the general flow of Figure 4b, note that one aspect of the present invention pertains to enhancing the timing of output by microROM circuit 148 immediately following a single clock instruction. Particularly, the present invention recognizes that once a given instruction is determined to be a single clock instruction, it is desirable to advance the next successive instruction through the pipeline as soon as possible. More specifically, when the instruction presently passing through a pipeline is a single clock instruction and, thus, requires only a single clock cycle for its EX stage, it is preferable to immediately advance the next successive instruction so that it follows only one stage behind the present single clock instruction. Figure 4c illustrates this preferred scenario. Particularly, Figure 4c illustrates a timing diagram including the instruction flow for two successive instructions through the x- and y-pipelines over the same clock cycles 150 through 156 shown in Figure 4b. Subscripts in Figure 4c illustrate the succession of the two instructions. The first x- and y-instructions, denoted with a subscript one, are single clock instructions and flow in the same manner as the instruction shown in Figure 4b. The second x- and y-instructions, denoted with a subscript two, are additional instructions which follow one stage immediately behind the first x- and y-instructions. Given the immediate succession of instruction flow, no stage is wasted, that is, no clock cycle occurs during the illustrated instruction flow where the instruction is not processed by either the ID1, ID2, AC1, AC2, or EX stage. The prior art is unable to achieve the preferred flow of Figure 4c for a single clock instruction. Figure 4d illustrates a timing diagram of the flow in the prior art for the two paired instructions of Figure 4c. Note that the microsequencer outputs for the second instruction, µsx 2 and µsy 2 , are output during clock cycle 154, rather than clock cycle 152 as shown in the inventive preferred flow of Figure 4c. Thus, clock cycle 152 is wasted in the prior art. Such a result occurs because of the prior art methodology in controlling the microsequencer circuit's output. Specifically, in the prior art, the microinstruction output by the microROM for a single clock instruction, µr, includes a control signal which subsequently (after a wasted clock cycle) causes the microsequencer circuit to output the next microROM microaddress, µs. More particularly, for each different microinstruction, a given bit or bits in µr indicate that an instruction is a single clock instruction. Only after this indication is the microsequencer caused to output the next microaddress, µs. This indication and delay is shown by an arrow in Figure 4d. As shown, at the end of 1 of clock cycle 152, the microinstruction, µrx1, indicates that its corresponding instruction is a single clock instruction. As described above, the next microaddress, µsx2, is output only after this indication and, thus, cannot occur until the beginning of clock cycle 154. Thus, in effect, the microsequencer must "wait on" the microROM output and, hence, at least one clock cycle is wasted during this waiting period. Note that the same delay occurs for the y-pipeline as shown by the delay arrow in Figure 4d between the microROM output µry1, and the responsive next output by the microsequencer, µsy2. In contrast to Figure 4d, however, the present invention includes a novel apparatus and methodology for achieving the preferred and improved flow of Figure 4c, and is described below. Figure 5 illustrates the block diagram of Figure 4a, but with additional details used to achieve the preferred timing of Figure 4c. Specifically, instruction decoder 21 includes separate registers 21x and 21y for the x- and y-instructions, respectively. Moreover, each separate register 21x and 21y is coupled to a respective programmable logic array (PLA). More particularly, PLAs 158x and 158y are coupled to the portions of registers 21x and 21y storing the opcode of each instruction. Note that while the opcode is illustrated as left-justified within each respective register 143x and 143y, such an illustration is arbitrary in that the opcode may be stored anywhere within the buffer and is not necessarily represented by contiguous bits therein. Each PLA 158x and 158y is further coupled to the control input of a respective microsequencer 146x and 146y. Lastly, while not illustrated, note that each PLA 158x and 158y may be further coupled to additional inputs to perform the functionality described below. The operation of the components of Figure 5 are best explained in the context of the timing diagram of Figure 4c. Assume first that each instruction register 21x and 21y stores two sequentially pipelined instructions, x 1 and x 2 , and y 1 and y 2 , respectively. Thus, as shown in Figure 4c, during 1 of the ID2 stage of clock cycle 150, microsequencer 146x receives control signals and outputs the next microaddress, µsx 1 . Moreover, during 2 of the ID2 stage of clock cycle 150, microsequencer 146y receives control signals and outputs the next microaddress, µsy 1 . Once µsx 1 and µsy 1 are output, they are applied to microROM circuit 148 to generate µrx 1 and µry 1 as described above. Returning to the ID2 stage occurring in clock cycle 150, however, note that each PLA 158x and 158y decodes the opcode of the respective instruction to determine whether the current instruction is a single clock instruction. In the preferred embodiment, the PLA is programmed to detect any possible pattern of opcode bits which comprise a single clock instruction. This detection may be based on a portion or all of the opcode bits. If the PLA detects a single clock instruction, the PLA controls its respective microsequencer to output the next microaddress, µs, during the immediately following clock cycle. For example, if PLA 158x determines that instruction x 1 is a single clock instruction, PLA 158x controls microsequencer 146x to output the next microaddress, µsx 2 , during the next clock cycle 152 as shown in Figure 4c. As another example, if PLA 158y determines that instruction y 1 is a single clock instruction, PLA 158y controls microsequencer 146y to output the next microaddress, µsy 2 , during the next clock cycle 152 as shown in Figure 4c. Note that each PLA 158x and 158y may analyze information in addition to selected opcode bits determine whether an instruction is a single clock instruction. For example, in an X86 architecture, other bits within the instruction may be analyzed, such as selected bits of the mod r/m byte. Moreover, in addition to opcode bits, bits outside the instruction may also be analyzed. For example, a bit indicating whether or not the protected mode is set also may be analyzed in determining whether an instruction is a single clock instructions. Other examples may become known to a person skilled in the art, and are intended as within the scope of the present invention. From the above, it should be appreciated that the configuration of Figure 5 eliminates the wasted clock cycle between successive microinstructions as possible in the prior art. Such an advance is extremely beneficial for a microprocessor using numerous single clock instructions because delay is reduced with respect to each of those instructions. As stated at the outset, instruction throughput is of primary importance and, as now shown, the present invention enhances such throughput. Note that it should be understood that a person skilled in the art may choose alternatives without necessarily departing from the inventive scope. For example, while the example discussed in connection with Figure 4c illustrates single clock instructions in both the x- and y-pipelines, the present invention also applies to a dual pipeline architecture when only one of the two pipelines is executing a single clock instruction (i.e., irrespective of the number of execution stages required by the instruction in the other pipeline). Thus, the invention also applies to other single or multiple pipeline architectures. As yet another example, while a PLA is shown monitoring opcode bits, structure other than a PLA may be used to detect the presence of a single clock instruction. As still another example, bits other than opcode bits may, given a particular architecture, be analyzed if representative of a single clock instruction. 4. Single microROM operation for supporting dual pipeline architecture. As mentioned in connection with Figures 4a-d, the processor architecture of the present invention includes a microROM circuit 148 which receives microaddresses, and produces microinstructions, in accordance with the timing diagram of Figure 4c. The present invention uses additional novel apparatus and methodology to achieve such timing while using a single microROM array. Particularly, Figure 6 illustrates such apparatus, and Figure 7 illustrates the detailed timing for the apparatus of Figure 6. With reference to Figure 6, microROM circuit 148 is shown in greater detail using blocks to represent various functionality. The blocks include a single microROM array 167 which services both the x- and y-pipelines. MicroROM array 167 includes a plurality of rows and columns for storing microinstructions which are addressed and output by the various blocks of Figure 6. With the exception of two components described below, the remainder are generally symmetric about microROM array 167; accordingly, the following description focuses on the x-related blocks, with corresponding reference numerals using the "x" designation. Components pertaining to the y-related blocks are likewise labeled, but use the "y" designation. The microaddress, µsx, is coupled to the input of a latch 152x which is clocked by 1. In the preferred embodiment, µsx is a fourteen bit address which is broken into separate components for addressing a row and column of microROM array 167, respectively. Specifically, microinstructions are stored in array 167 such that sixteen instructions are stored along a row. Thus, once a row is selected, one of sixteen columns from that row is further selected to choose the particular microinstruction from the many stored along the row. In particular, ten bits of µsx are coupled from latch 152 to the input of a row decode block 154x. Similarly, four bits of µsx are coupled from latch 152 to the input of a column decode block 156x. While the groups of ten bits are shown as the first ten bits of µsx (i.e., [0:9]) and the last four bits of µsx (i.e., [10:13]), it should be understood that non-contiguous bits of µsx may be selected for either row or column decoding. It also should be understood that the microaddresses, µsx and µsy, may consist of more or less than fourteen bits. The output of row decode block 154x is coupled to the input of a latch 158x clocked by 2. Further, the output of latch 158x is coupled to the input of a tristate driver 160x clocked by 1. Similarly, the output of column decode block 156x is coupled to the input of a latch 162x, which is clocked by 2. The output of tristate driver 160x is coupled to an address bus denoted 164xy suggesting, as discussed below, that either x- or y-addresses may be communicated on the bus. Indeed, note that the output of tristate driver 160y is also coupled to address bus 164xy. Address bus 164xy is coupled to the input of row driver 166xy, which is further coupled to microROM array 167. The circuitry symmetry in Figure 6 creates componentry for the y-row related circuitry similar to that just described in connection with x-row related circuitry. Thus, bits [0:9] of µsy are coupled to a row decode block 154y which has an output coupled to the input of a latch 158y. The output of latch 158y is coupled to the input of a tristate driver 160y, which has an output coupled to address bus 164xy. Note, however, that although the data path for the y-row circuitry is similar to the x-row circuitry, the clock signals for latch 158y and driver 160y are switched to achieve the timing function described below. Returning to the x-column related circuitry, the output of latch 162x is coupled to the control inputs of a sense amplifier, latch and multiplexer block 168x. Note that block 168x is capable of performing each of the functions of its name. Particularly, the data input of block 168x is coupled to an output of microROM array 167. Moreover, block 168x is clocked by 1 to performing a latching function. Lastly, because the output of latch 162x is coupled to the control input of block 168x, block 168x latches only selective portions (i.e., a particular column) of a row of data output by microROM array 167 as discussed in greater detail below. The output of block 168x is coupled to the input of a flip flop 170x which is clocked by 1. The output of flip flop 170x is coupled to any line, bus, or architecture which requires access to the x-microinstruction produced by microROM array 167. The circuitry symmetry in Figure 6 creates similar componentry for the y-column related circuitry. Thus, bits [10:13] of µsy are coupled to a column decode block 156y which is further coupled to the input of a latch 162y. The output of latch 162y is coupled to the control input of a sense amp, latch and multiplexer block 168y. Again, the y-column data path is therefore generally the same as they x-row data path, but the clocking signals are reversed to achieve the timing described below. Note also, however, that the y-row data path differs slightly in one regard, namely, the output of multiplexer block 168y is coupled to the input of a latch 172y. This is in contrast to the output of multiplexer block 168x which is coupled to a flip flop rather than a latch. This difference also accommodates the timing functionality described below. Moreover, the output of latch 172y is coupled to any line, bus, or architecture which requires access to the y-microinstruction produced by microROM array 167. As shown in Figure 4c, one invention of the preferred architecture involves two accesses of microROM circuit 148 in the same clock cycle 150, but in separate phases, 1 and 2. Further, by applying µrx and µry to microROM circuit 148 in a single clock cycle, µrx is fully available by the end of one clock cycle after µsx and, similarly, µry is fully available by the end of one clock cycle after µsy. The blocks of Figure 6 permit such operation, and are best understood by the more detailed timing diagram of Figure 7. Figure 7 illustrates a detailed timing diagram for both the x- and y-accessing of microROM array 167 during the clock cycles 150 through 154, similar to that shown in Figure 4b. For purposes of clarity, the operation in connection with the x-pipeline (i.e., x-microaddress and x-microinstruction) is described first and illustrated along the leftmost portion of the diagonal flow of the figure. Similarly, the operation in connection with the y-pipeline (i.e., y-microaddress and y-microinstruction) is described second and illustrated along the rightmost portion of the figure. With reference to the leftmost portion of Figure 7, µsx 1 is latched by latch 152x by the close of 1 of clock cycle 150. Accordingly, the appropriate bits of µ sx 1 are provided to row and column decode blocks 154x and 156x, respectively. Each of these decode blocks decodes the bits according to principles known in the art to identify the corresponding row and column within microROM array 167 which are addressed by µsx 1 . Thus, the microaddress decode occurs during 2 of clock cycle 150, and resulting row and column decoded signals are latched at the close of 2 of clock cycle 150 by latches 158x and 162x, respectively. Note the decoded column signal output by latch 162x is immediately coupled to the control input of multiplexer block 168x. Next, with the immediately following rising transition of 1 during clock cycle 152, tristate driver 160x drives address bus 164xy with the decoded row value latched in latch 158x. This value is received by driver 166xy which, in accordance with principles known in the art, provides a sufficient driving signal(s) to the selected row of microROM array 167. Thus, during 1 of clock cycle 152, the driving operation and response by microROM array 167 generates and operates the appropriate signals of the addressed row. At the falling edge of 1 of clock cycle 152, multiplexer 168x latches selected bits of the row of signals output by microROM array 167. Particularly, certain bits (i.e., a column which stores the desired microinstruction,) are selected in response to the decoded column address received from latch 162x and latched to the output of multiplexer 168x. Note that this event also corresponds to the end of 1 during clock cycle 152 shown in Figure 4b, above. During 2 of clock cycle 152, flip flop 170x samples its input and, upon the falling edge of 2, flip flop 170x outputs the sampled input. Accordingly, at the end of 2 of cycle 152, the x-pipeline microinstruction, µrx, is available to the remainder of the processor for operation of the AC2 stage described above. Note also that flip flop 170x is included to isolate the current microinstruction from any potential changes occurring in the output of microROM array 167 or multiplexer 168x. In other words, after the falling edge of 2 of clock cycle 152, any change in the output of either microROM array 167 or multiplexer 168x does not affect the output of flip flop 170x during the next phase, 1 of cycle 154. Thus, any such change will not affect the components of the processor examining the current µrx microinstruction. Having described the operation of the x-microaddressing and x-microinstruction issuing methodologies, reference is now made to the rightmost portion of Figure 7 to illustrate the contemporaneous operations with respect to the operations of issuing the y-microaddressing and y-microinstruction. First, µ sy 1 is latched by latch 152y by the close of 2 of clock cycle 150. Accordingly, the appropriate bits of µsy 1 are provided to row and column decode blocks 154y and 156y, respectively. Like the x-bits, but one phase later, the y-bits are decoded during 2 of clock cycle 150 to identify the corresponding row and columns within microROM array 167 which are addressed by µsy 1 . Moreover, the resulting row and column decoded signals are latched at the close of 1 of clock cycle 152 by latches 158y and 162y, respectively. Next, with the immediately following rising transition of 2 during clock cycle 152, tristate driver 160y drives address bus 164xy with the decoded row value latched in latch 158y. Note that tristate driver 160x is in a high impedance mode during this time so that only the y-address is communicated to the address bus 164xy. Again, this value is received by driver 166xy which drives the selected row of microROM array 167. Thus, during 2 of clock cycle 152, the driving operation and response by microROM array 167 generates and outputs the appropriate signals of the addressed row. At the falling edge of 2 of clock cycle 152, multiplexer 168y latches selected bits (i.e., a column) of the row of signals output by microROM array 167. In this instance, these bits, which represent the desired microinstruction, are selected in response to the decoded column address received from latch 162y and latched to the output of multiplexer 168y. Note that this event also corresponds to the end of 2 during clock cycle 152 shown in Figure 4b, above. During 1 of clock cycle 154, latch 172y becomes transparent so that the signals at its inputs become available at its outputs. Thus, at the rising edge of 1 of clock cycle 154, the remainder of the processor has access to the y-pipeline microinstruction, µry. Note that latch 172y, like that flip flop 170x, is included to isolate the current microinstruction from any potential changes occurring in the output of microROM array 167 or multiplexer 168y. Thus, at the end of 1 of cycle 154, any change in the output of either microROM array 167 or multiplexer 168y does not affect the output of latch 172y during the next phase, 2 of cycle 154. Thus, any such change will not affect the components of the processor examining the current µry microinstruction. From the above, it should be appreciated that the functional blocks set forth in Figure 6 permit the preferred timing shown in Figure 7. Thus, using separate phases, a single microROM array 167 may be accessed twice in a single clock cycle, while still providing microinstructions corresponding to each access within one full clock cycle of the access. Thus, independent x- and y-pipelines may share the same single array without sacrificing throughput Note that it should be understood that a person skilled in the art may choose alternatives without necessarily departing from the inventive functionality and structure described. For example, clock widths may be adjusted so that more than two phases occur in what is deemed a "clock cycle," while still maintaining the ability for multiple pipelines to share the same microROM array. This example, as well as others apparent to one skilled in the art, fall within the scope of the invention. 5. Expediting Execution of Condition Dependent Instructions. As discussed in connection with Figure 4a, the preferred embodiment includes a microsequencer circuit 146 which operates as a sophisticated multiplexer to choose between various microaddresses at its inputs and output the selected microaddress for addressing microROM circuit 148. In connection with microsequencer circuit 146, the present invention further includes apparatus and methodology for expediting the execution of condition dependent instructions. In the preferred embodiment, one example of such a condition dependent instruction is a change of flow ("COF") instructions. Such COF instructions are known in the art and, include, by way of example in the X86 architecture, instructions such as FAR JUMP, FAR CALL, and IRET. The preferred apparatus and methodology for expediting such COF instructions, as well as other condition dependent instructions, are described in connection with Figure 8, immediately below. Figure 8 illustrates a portion of the components of Figure 4a, with additional apparatus for expediting condition dependent instructions. For the remainder of this discussion, the preferred embodiment is discussed, by way of example, as applying to COF instructions. It should be understood, however, that the COF instruction is by way of example, and is not intended to limit the scope of the invention as defined by its claims. Therefore, continuing with Figure 8, it again illustrates microsequencer circuit 146 receiving microaddresses and, via various control signals, providing microaddresses µsx and µsy to microROM circuit 148. Particularly, however, apparatus in addition to Figure 4a is shown for providing microaddresses and control in connection with COF instructions. Specifically, Figure 8 further illustrates a COF instruction decode circuit 174. COF instruction decode circuit 174 includes various data inputs, preferably including an input 176a for receiving descriptors, an input 176b for receiving mode bits, and an input 176c for receiving "history bits." History bits include selected bits stored from previously issued instructions and/or bits from previously executed microinstructions corresponding to the present instruction being executed. COF instruction decode circuit further includes control inputs 178x and 178y for receiving control signals corresponding to the respective x- and y-instructions. For purposes more readily apparent below, these control signals are referred to herein as the x-Multiway and y-Multiway control signals, respectively. COF instruction decode circuit further includes an output 180xy, which is illustrated as splitting into respective x- and y-outputs 180x and 180y. Output 180x is coupled to provide a SECONDARY ADDRESS to the lower portion of an address register 182x, or a DONE signal to control microsequencer 146x. Similarly, output 180y is coupled to provide the SECONDARY ADDRESS to the lower portion of an address register 182y, or the DONE signal to control microsequencer 146y. The upper portion of address register 182x is coupled to the x-output of microROM circuit 148 and, thus, is capable of receiving a portion of the x-microinstruction, µrx. As described in greater detail below, this connection permits µrx to provide a BASE ADDRESS to the upper portion of address register 182x, Similarly, the upper portion of address register 182y is coupled to the y-output of microROM circuit 148 and, thus, is capable of receiving a portion of the y-microinstruction, µry. Thus, also as described in greater detail below, this connection permits µry to provide a BASE ADDRESS to the upper portion of address register 182y. Before proceeding with the operation of the apparatus of Figure 8, a discussion of at least one known prior art technique for handling COF instructions, or other types of conditional dependent instructions, is instructive. This discussion is further described using the microinstructions of Table 1, with it understood that such microinstructions are provided solely by way of example to facilitate an understanding of the known prior art. In the known prior art, a COF instruction would commence its execution stage by addressing a first microinstruction within the microROM. For the COF instruction, after its first, or subsequent microinstructions, a series of sequential conditional microinstruction JUMPs (CJUMP) are typically encountered, wherein each of those CJUMPs tested various conditions. Thus, referring to Table 1, consider the instance where a COF instruction commenced by addressing the first of the four successive microinstructions located at address 1 of the microROM: 1 CJUMP condition #1 10 2 CJUMP condition #2 11 3 CJUMP condition #3 12 4 CJUMP condition #4 13 5 DONE microinstruction address microinstruction destination if condition met In the prior art, if condition #1 were met, the microROM would output address 10, which is the next (or "destination") microinstruction address within itself for continuing the execution of the COF instruction. The process flow would then sequence by jumping to the microinstruction at location 10, and continue until the functionality for the COF instruction was completed. If condition #1 were not met, however, then a counter was used to increment a portion of the microaddress which identified the then-executing microinstruction. Thus, the incremented value would identify address 2 and so the sequence would address the microinstruction at that address. Again, the successive microinstruction would test another condition, namely, condition #2. The process would repeat until a condition was met, or all conditions in the succession were tested with no satisfaction. If no conditions were met, the microROM would issue a DONE signal, thereby indicating that the COF instruction had completed its execution cycle, and the instruction entry point into the microROM for the next instruction could be accessed. Note that the above example uses sequential numbers only to facilitate an understanding of the concept, but is not intended to indicate that the conditional microinstructions are necessarily addressed in an integer incremental fashion. From Table 1 and its corresponding description, one skilled in the art may readily appreciate that numerous CJUMPs may be necessary before a condition corresponding to the COF instruction (or other conditional dependent instruction) being executed is satisfied. Typically, each testing of a condition and issuance of the next condition to be tested expends at least one clock cycle. Thus, if numerous conditions are unsatisfied for a given COF instruction, numerous clock cycles are expended in executing that COF instruction. In the preferred embodiment of the present invention, however, and as described immediately below, such conditional testing is reduced to a single clock cycle, with the clear benefit of reducing the number of potential clock cycles and, thus, enhancing overall processing speed. Having described the prior art example above, reference is made to the operation of the apparatus of Figure 8. When the COF (or other condition dependent) instruction is received by the instruction decoder (see, e.g., Figure 4a), its execution stage commences by issuing a control signal from microROM circuit 148. For purposes of explanation, this signal is identified herein as "Multiway," indicating that flow can be directed to multiple, differing destinations (i.e., ways) in the next successive clock cycle. Thus, if the COF instruction is executing in the x-pipeline, an x-Multiway control signal is output by microROM circuit 148 as part of the x-microinstruction. The x-Multiway control signal controls COF instruction decode circuit 174 as described below. The x-microinstruction also simultaneously includes a BASE ADDRESS which loads into the upper portion of address register 182x. COF instruction decode circuit 174 preferably includes combinatorial logic which analyzes the preferred information at its inputs 176a, 176b, and 176c, and determines a SECONDARY ADDRESS in response to those inputs. In the preferred embodiment, the SECONDARY ADDRESS is a four bit signal and, thus, is capable of identifying up to sixteen (i.e., 2 4 = 16) specific addresses. Moreover, the SECONDARY ADDRESS is preferably calculated based on the input information before the issuance of the Multiway control signal. Thereafter, as in the example of the x-Multiway control signal above, issuance of the Multiway control signal causes the already-determined SECONDARY ADDRESS to load from COF instruction decode circuit 174 into the lower portion of address register 182x. At this point, both the lower and upper portions of address register 182x are loaded with address portions. Those portions are then combined, preferably by concatenation, to form a single DESTINATION ADDRESS. This DESTINATION ADDRESS is the location within microROM circuit 148 for proceeding with the execution of the COF instruction. Accordingly, the DESTINATION ADDRESS is coupled to microsequencer 146x which selects it and couples it to microROM circuit 148, thereby addressing and executing the next microinstruction of the execution stage for the COF instruction. Note that the operation described in connection with the x-pipeline applies equally to the y-pipeline. Thus, at the same time that decode circuit 174 outputs it SECONDARY ADDRESS to the lower portion of address register 182x, the same SECONDARY ADDRESS is output to the lower portion of address register 182y. For the y-pipeline, a different BASE ADDRESS is provided from microROM circuit 148, and this different BASE ADDRESS combines with the SECONDARY ADDRESS in the same manner, and for the same purpose, as described in connection with the x-pipeline. Similarly, the DONE signal, described in further detail below, is also coupled simultaneously from decode circuit 174 to both microsequencer 146x and 146y. From the above, it therefore should be appreciated that the embodiment of Figure 8 removes the necessity to exhaust numerous CJUMPs, and resulting clock cycles, in advancing the execution of a COF or other condition dependent instruction. Particularly, the preferred embodiment requires only a single clock cycle, whereas the prior art required multiple clock cycles, to determine the destination address for executing the COF instruction. Naturally, therefore, throughput is enhanced, particularly with applications involving numerous COF instructions. As described briefly above, COF instruction decode circuit 174 may provide a DONE signal to control microsequencer 146x in lieu of providing the SECONDARY ADDRESS to address register 182x as detailed above. This same functionality applies to the y-components and, hence, COF instruction decode circuit 174 may provide a DONE signal to control microsequencer 146y in lieu of providing the SECONDARY ADDRESS to address register 182y. This feature also provides advantages over the prior art. Again, before detailing the DONE signal aspect of the present invention, a reminder of the known prior art technique for indicating the completion of execution of a COF or other condition dependent instruction is instructive. Recall from the description above that the known prior art methodology issued a DONE signal from the microROM. Typically, this occurred only after all other conditions in a sequence were tested, or after an alternative routine performed its steps and addressed the microROM so that it would issue a DONE signal. Moreover, the known prior art further required a one-clock advanced notice that the COF instruction was complete and, hence, two clock cycles were expended in providing an indication that execution of the COF instruction was DONE. In contrast, the present invention reduces the number of clock cycles in indicating a completion of the COF instruction. Accordingly, successive instructions may be advanced through the pipeline and, thus, throughput is enhanced. The specific implementation of the DONE signal generation in accordance with the present invention is as follows. Again, COF instruction decode circuit 174 operates independently of accesses to microROM circuit 148. Moreover, in the present embodiment, because of the various input data available to COF instruction decode circuit 174, it determines when a COF instruction has completed its execution stages. Thus, when the Multiway control signal is received by COF instruction decode circuit 174, in lieu of a SECONDARY ADDRESS, it outputs a DONE signal if the combination of its inputs indicates completion of the execution of the COF instruction. In the preferred embodiment, the DONE signal is encoded in the same four bit output from decode circuit 174 discussed above in connection with the SECONDARY ADDRESS. Such a configuration permits dual use of the same conductors without an independent output. One skilled in the art, however, may readily appreciate that a separate output could be provided, with more or less bits, for communicating the DONE signal. The DONE signal from COF instruction decode circuit 174 is coupled to microsequencer circuit 146 and controls the microsequencer circuit 146 such that the next microaddress it selects is the entry point microaddress to the instruction following the COF instruction. Accordingly, one skilled in the art may appreciate that the present invention immediately indicates the completion of the COF instruction without requiring the microROM to indicate such status. Thus, one or more clock cycles are saved over the known prior art embodiment described above. Note that while the present invention has been described in detail, numerous alterations, modifications, or substitutions could be made hereto. For example, various techniques and/or architectures for calculating the SECONDARY ADDRESS and DONE signals are readily appreciated by a person skilled in the art, given an understanding of the specific microprocessor architecture, instruction set, and the need to jump to various different locations within microROM circuit 148 based on the satisfaction of various conditions. Indeed, the input data to decode circuit 174 may be altered to accommodate a change in the COF instruction set and/or the desired resultant address(es) based on those instructions and the conditions tested by such instructions. The same applies to the formation of the DONE signal. Moreover, as stated in numerous instances above, the present invention may also extend to conditional dependent instructions other than a COF instruction. Any derivations of these inputs or instructions, as well as other modifications or substitutions to the invention, should not affect the scope of the intended invention. 6. Conclusion. Note that although the preferred embodiment has been disclosed, various alternatives may be made by a person skilled in the art without departing from the invention as claimed. Numerous examples of such alternatives have been set forth throughout. As another example, while block diagrams have been shown herein, it should be understood that such blocks represent the methodology of the invention and, thus, do not necessarily limit its scope; hence, various functions may be moved to other defined areas of the microprocessor without necessarily departing from the scope of the invention. As yet a final example, while the architecture and timing diagrams involve a dual-pipeline scenario, various aspects of the present invention may be applied to a single pipeline, or other multiple-pipeline, processor architecture. The invention encompasses any modifications or additions to the described embodiments and alternative embodiments that fall within the scope thereof.

Claims: A processor comprising: a decoder (21x) for storing instructions to be decoded; a memory (148) responsive to a micro-address to output a micro-instruction associated with an instruction stored in said decoder (21); and sequencer circuitry (146x) coupled to provide a micro-address to said memory (148); characterised by a detector (158x) coupled to the decoder (21x) and operable to detect whether or not an instruction comprises a single clock instruction before said memory (148) outputs the micro-instruction associated with that instruction, said detector (158x) being responsive to detection of a single clock instruction to input a control signal to the sequencer circuitry (146x) to cause the sequencer circuitry to issue, independently of said memory (148), another micro-address associated with another instruction. A processor according to claim 1, comprising: second sequencer circuitry (146y) coupled to provide a micro-address to cause said memory (148) to output a micro-instruction associated with a further instruction stored by said decoder (21); and a second decoder (21y) coupled to receive instructions from said decoder (21x), a second detector (158y) to detect whether or not said further instruction comprises a single clock instruction before said memory (148) outputs the micro-instruction associated with that further instruction, said second detector (158y) being responsive to detection of a single clock instruction to input a control signal to the second sequencer circuitry (146y) to cause the second sequencer circuitry to issue, independently of said memory (148), another micro-address associated with another instruction. A processor according to claim 2, wherein said decoders (21x, 21y) comprise respective registers. A processor according to any one of claims 1 to 3, wherein the or each said detector (158x, 158y) is responsive to detection of a single clock instruction to input a control signal to its associated sequencer circuitry (146x, 146y) to cause that sequencer circuitry to issue said another micro-address in the next following clock cycle. A processor according to any one of claims 1 to 4, wherein the or each said detector (158x, 158y) includes a programmable logic array. A processor according to claim 5, wherein the or each said programmable logic array detects selected bits of an instruction to detect whether or not said instruction is a single clock instruction determination. A processor according to any one of the preceding claims, wherein the or each said detector (158x, 158y) is responsive to a plurality of opcode bits of instructions stored in said memory (148) to detect whether or not an instruction is a single clock instruction. A processor according to claim 7, wherein the or each said detector (158x, 158y) is also responsive to a protected mode bit external from an instruction to detect whether or not an instruction is a single clock instruction. A processor according to any one of the preceding claims, wherein the memory (148) is a micro-ROM. A method of operating a processor, the method comprising the steps of: storing instructions; outputting a micro-address from a sequencer (146x) to address a memory (148) thereby causing the memory (148) to issue a micro-instruction associated with a stored instruction; characterised by detecting, before said memory (148) outputs the micro-instruction associated with an instruction, whether or not that instruction is a single clock instruction; and in response to the detection of a single clock instruction, inputting a control signal to the sequencer (146x) to provide, independently of said memory (148), another micro-address associated with another instruction. A method according to claim 10, further characterised by storing further instructions; and, substantially in parallel with implementation of the method according to claim 10, outputting a micro-address from a further sequencer (146y) to address said memory (148), thereby causing the memory to issue a micro-instruction associated with a stored further instruction; detecting, before said memory (148) outputs the micro-instruction associated with said further instruction, whether or not that further instruction is a single clock instruction; and, in response to the detection of a single clock further instruction, inputting a control signal to the further sequencer (146y) to issue, independently of said memory (148), another micro-address associated with another instruction. A method according to claim 10 or 11, wherein said detection is carried out by logical decoding of a preselected portion of an instruction to detect whether or not said instruction is a single clock instruction. A method according to any one of claims 10 to 12, wherein, in response to detection of a single clock instruction, the control signal input to one of the sequencer (146x) and the further sequencer (146y) causes the issue of another micro-address associated with another instruction in the next succeeding clock cycle. A method according to any one of claims 10 to 13, further characterised by repeating said method for a series of instructions.

Inventors: BLUHM MARK, EITRHEIM JOHN K, GARIBAY RAUL A JR, HERVIN MARK W, MCMAHAN STEVEN C, MCMAHON RONALD S, BLUHM, MARK, EITRHEIM, JOHN K., GARIBAY, RAUL A. JR., HERVIN, MARK W., MCMAHAN, STEVEN C., MCMAHON, RONALD S.
Applicants: NAT SEMICONDUCTOR CORP, NATIONAL SEMICONDUCTOR CORPORATION
Application year: 1994
Main classification: G06F9
IPC codes: G06F9